metabolism of carbohydartes and biochemistry of carbohydrates and proteins

1. NAIHA ATTA
ROLL NO. 14211507-022
METABOLISM AND ROLE OF CARBOHYDRATES
PROTEINS AND BIOCHEMISTRY OF PROTEINS

2. METABOLISM OF CARBOHYDRATES
All digestible forms of carbohydrates are eventually transformed into glucose, it is important to
consider how glucose is able to provide energy in the form of adenosine triphosphate (ATP) to
various cells and tissues. Glucose is metabolized in three stages:
 glycolysis
 the Krebs Cycle
 oxidative phosphorylation
During exercise, hormonal levels shift and this disruption of homeostasis alters the metabolism of
glucose and other energy-bearing molecules.
GLYCOLYSIS
 The breakdown of glucose to provide energy begins with glycolysis.
 Glucose enters the cytosol of the cell, or the fluid inside the cell not including cellular
organelles. Next, glucose is converted into two, three-carbon molecules of pyruvate
through a series of ten different reactions.
 A specific enzyme catalyzes each reaction along the way and a total of two ATP are
generated per glucose molecule.
 Since ADP is converted to ATP during the breakdown of the substrate glucose, the process
is known as substrate-level phosphorylation.
 During the sixth reaction, glyceraldehyde 3-phosphate is oxidized to 1,3
bisphosphoglycerate while reducing nicotinamide adenosine dinucleotide (NAD) to
NADH, the reduced form of the compound. NADH is then shuttled to the mitochondria of
the cell where it is used in the electron transport chain to generate ATP via oxidative
phosphorylation.

3.  The most important enzyme in glycolysis is called phosphofructokinase (PFK)and
catalyzes the third reaction in the sequence. In other words, glucose will be completely
degraded to pyruvate after this reaction has taken place.

4. KREB CYCLE
 There are many compounds that are formed and recycled during the Krebs Cycle (Citirc
Acid Cycle). These include oxidized forms of nictotinamide adenine dinucleotide (NAD+)
and flavin adenine dinucleotide (FAD) and their reduced counterparts: NADH and
FADH2. NAD+ and FAD are electron acceptors and become reduced while the substrates
in the Krebs Cycle become oxidized and surrender their electrons.
 The Krebs Cycle begins when the pyruvate formed in the cytoplasm of the cell during
glycolysis is transferred to the mitochondria, where most of the energy inherent in glucose
is extracted.
 In the mitochondria, pyruvate is converted to acetyl CoA by the enzyme pyruvate
carboxlase.
 In general, Acetyl-CoA condenses with a four carbon compound called oxaloacetate to
form a six carbon acid.
 This six-carbon compound is degraded to a five and four carbon compound, releasing two
molecules of carbon dioxide.
 At the same time, two molecules of NADH are formed. Finally, the C-4 carbon skeleton
undergoes three additional reactions in which guanosine triphosphate (GTP), FADH2 and
NADH are formed, thereby regenerating oxaloacetate.
 FADH2 and NADH are passed on to the electron transport chain that is embedded in the
inner mitochondria membrane. GTP is a high-energy compound that is used to regenerate
ATP from ADP. Therefore, the main purpose of the Krebs Cycle is to provide high-energy
electrons in the form of FADH2 and NADH to be passed onward to the electron transport
chain.

5. ELECTRON TRANSPORT CHAIN
 The high-energy electrons contained in NADH and FADH2 are passed on to a series of
enzyme complexes in the mitochondrial membrane.
 Three complexes work in sequence to harvest the energy in NADH and FADH2 and
convert it to ATP: NADH-Q reductase, cytochrome reductase and cytochrome oxidase.
 The final electron acceptor in the electron transport chain is oxygen.

6.  Each successive complex is at lower energy than the former so that each can accept
electrons and effectively oxidize the higher energy species.
 In effect, each complex harvests the energy in these electrons to pump protons across the
inner mitochondria membrane, thereby creating a proton gradient.
 In turn, this electropotential energy is converted to chemical energy by allowing proton
flux back down its chemical gradient and through specific proton channels that synthesize
ATP from ADP.
 Approximately two molecules of ATP are produced during the Kreb cycle reactions, while
approximately 26 to 30 ATP are generated by the electron transport chain.
 In summary, the oxidation of glucose through the reduction of NAD+ and FADH is coupled
to the phosphorylation of ADP to produce ATP. Hence, the process is known as oxidative
phosphorylation.

7. ROLE OF CARBOHYDRATES
Carbohydrates have major functions within the body:
1) Providing energy and regulation of blood glucose
2) Sparing the use of proteins for energy
3) Biological recognition processes
4) Flavor and Sweeteners
5) Dietary fiber
PROVIDING ENERGY
 The primary role of carbohydrates is to supply energy to all cells in the body. Many cells
prefer glucose as a source of energy versus other compounds like fatty acids. Some cells,
such as red blood cells, are only able to produce cellular energy from glucose.
 About 70 percent of the glucose entering the body from digestion is redistributed (by the
liver) back into the blood for use by other tissues. Cells that require energy remove the
glucose from the blood with a transport protein in their membranes.
 The energy from glucose comes from the chemical bonds between the carbon atoms..
SPARING THE USE OF PROTEINS FOR ENERGY
 The processes of protein degradation and ketosis can create problems of their own during
prolonged fasting, they are adaptive mechanisms during glucose shortages.
 The first priority of metabolism during a prolonged fast is to provide enough glucose for
the brain and other organs that dependent upon it for energy in order to spare proteins for
other cellular functions.
 The next priority of the body is to shift the use of fuel from glucose to fatty acids and
ketone bodies. From then on, ketones become more and more important as a source of fuel
while fatty acids and glucose become less important.

8. FLAVOUR AND SWEETNERS
 A less important function of carbohydrates is to provide sweetness to foods.
 Receptors located at the tip of the tongue bind to tiny bits of carbohydrates and send what
humans perceive as a "sweet" signal to the brain.
 However, different sugars vary in sweetness.
 For example, fructose is almost twice as sweet as sucrose and sucrose is approximately
30% sweeter than glucose.
DIETARY FIBER
 Dietary fibers such as cellulose, hemicellulose, pectin, gum and mucilage are important
carbohydrates for several reasons.
 Soluble dietary fibers like pectin, gum and mucilage pass undigested through the small
intestine and are degraded into fatty acids and gases by the large intestine.
 The fatty acids produced in this way can either be used as a fuel for the large intestine or
be absorbed into the bloodstream. Therefore, dietary fiber is essential for proper intestinal
health.
BIOLOGICAL RECOGNITION PROCESSES
 Carbohydrates not only serve nutritional functions, but are also thought to play important
roles in cellular recognition processes.
 For example, many immunoglobulins (antibodies) and peptide hormones contain
glycoprotein sequences. These sequences are composed of amino acids linked to
carbohydrates.
 During the course of many hours or days, the carbohydrate polymer linked to the rest of
the protein may be cleaved by circulating enzymes or be degraded spontaneously.

9. PROTEINS
INTRODUCTION
 Proteins are polypeptides, which are made up of many amino acids linked together as a
linear chain.
 The structure of an amino acid contains a amino group, a carboxyl group, and a R group
which is usually carbon based and gives the amino acid it's specific properties.
 These properties determine the interactions between atoms and molecules, which are: van
der Waals force between temporary dipoles, ionic interactions between charged groups,
and attractions between polar groups.
 Proteins form the very basis of life. They regulate a variety of activities in all known
organisms, from replication of the genetic code to transporting oxygen, and are generally
responsible for regulating the cellular machinery and determining the phenotype of an
organism.
 Proteins accomplish their tasks in the body by three-dimensional tertiary and quaternary
interactions between various substrates.

10. BIOCHEMISTRY OF PROTEIN
 Protein, highly complex substance that is present in all living organisms. Proteins are of
great nutritional value and are directly involved in the chemical processes essential for life.
The importance of proteins was recognized by the chemists in the early 19th century who
coined the name for these substances from the Greek proteios, meaning “holding first
place.”
 Proteins are species-specific; that is, the proteins of one species differ from those of another
species. They are also organ-specific; for instance, within a single organism, muscle
proteins differ from those of the brain and liver.
 A protein molecule is very large compared with molecules of sugar or salt and consists of
many amino acids joined together to form long chains, much as beads are arranged on a
string.
 There are about 20 different amino acids that occur naturally in proteins. Proteins of similar
function have similar amino acid composition and sequence.
The amino acid composition of proteins
 Although more than 100 amino acids occur in nature, particularly in plants, only 20 types
are commonly found in most proteins.
 In protein molecules the α-amino acids are linked to each other by peptide bonds between
the amino group of one amino acid and the carboxyl group of its neighbor
 It is customary to write the structure of peptides in such a way that the free α-amino group
(also called the N terminus of the peptide) is at the left side and the free carboxyl group
(the C terminus) at the right side.
 Proteins are macromolecular polypeptides—i.e., very large molecules composed of many
peptide-bonded amino acids.

11.  Most of the common ones contain more than 100 amino acids linked to each other in a long
peptide chain. The average molecular weight (based on the weight of a hydrogen atom as
1) of each amino acid is approximately 100 to 125; thus, the molecular weights of proteins
are usually in the range of 10,000 to 100,000 daltons (one dalton is the weight of one
hydrogen atom).
 The species-specificity and organ-specificity of proteins result from differences in the
number and sequences of amino acids. Twenty different amino acids in a chain 100 amino
acids long can be arranged in far more than 10100 ways (10100 is the number one followed
by 100 zeroes).
Physicochemical properties of the amino acids
 The physicochemical properties of a protein are determined by the analogous properties of
the amino acids in it.
 The α-carbon atom of all amino acids, with the exception of glycine, is asymmetric; this
means that four different chemical entities (atoms or groups of atoms) are attached to it.
As a result, each of the amino acids, except glycine, can exist in two different spatial, or
geometric, arrangements (i.e., isomers), which are mirror images akin to right and left
hands.
 These isomers exhibit the property of optical rotation. Optical rotation is the rotation of the
plane of polarized light, which is composed of light waves that vibrate in one plane, or
direction, only.
 In bacteria, D-alanine and some other D-amino acids have been found as components of
gramicidin and bacitracin. These peptides are toxic to other bacteria and are used in
medicine as antibiotics. The D-alanine has also been found in some peptides of bacterial
membranes.
LEVELS OF STRUCTURAL ORGANIZATION IN PROTEINS
PRIMARY STRUCTURE

12.  Analytical and synthetic procedures reveal only the primary structure of the proteins—that
is, the amino acid sequence of the peptide chains. They do not reveal information about the
conformation (arrangement in space) of the peptide chain—that is, whether the peptide
chain is present as a long straight thread or is irregularly coiled and folded into a globule.
 The configuration, or conformation, of a protein is determined by mutual attraction or
repulsion of polar or nonpolar groups in the side chains (R groups) of the amino acids.
 The former have positive or negative charges in their side chains; the latter repel water but
attract each other. Some parts of a peptide chain containing 100 to 200 amino acids may
form a loop, or helix; others may be straight or form irregular coils.
 The primary structure of a protein is determined by its amino acid sequence without any
regard for the arrangement of the peptide chain in space.
SECONDARY STRUCTURE
 The secondary structure is determined by the spatial arrangement of the main peptide chain
without any regard for the conformation of side chains or other segments of the main chain.
 The nitrogen and carbon atoms of a peptide chain cannot lie on a straight line, because of
the magnitude of the bond angles between adjacent atoms of the chain; the bond angle is
about 110°. Each of the nitrogen and carbon atoms can rotate to a certain extent, however,
so that the chain has a limited flexibility.

13.  Because all of the amino acids, except glycine, are asymmetric L-amino acids, the peptide
chain tends to assume an asymmetric helical shape; some of the fibrous proteins consist of
elongated helices around a straight screw axis. Such structural features result from
properties common to all peptide chains. The product of their effects is the secondary
structure of the protein.
TERTIARY STRUCTURE
 The tertiary structure is determined by both the side chains and other adjacent segments of
the main chain, without regard for neighbouring peptide chains
 The tertiary structure is the product of the interaction between the side chains (R) of the
amino acids composing the protein.
 Some of them contain positively or negatively charged groups, others are polar, and still
others are nonpolar. The number of carbon atoms in the side chain varies from zero in
glycine to nine in tryptophan.
 Positively and negatively charged side chains have the tendency to attract each other; side
chains with identical charges repel each other.
 The bonds formed by the forces between the negatively charged side chains of aspartic or
glutamic acid on the one hand, and the positively charged side chains of lysine or arginine
on the other hand, are called salt bridges.

14.  Mutual attraction of adjacent peptide chains also results from the formation of numerous
hydrogen bonds.
QUATENARY STRUCTURE
 Finally, the term quaternary structure is used for the arrangement of identical or different
subunits of a large protein in which each subunit is a separate peptide chain.
 The nature of the quaternary structure is demonstrated by the structure of hemoglobin. Each
molecule of human hemoglobin consists of four peptide chains, two α-chains and two β-
chains; i.e., it is a tetramer.
 The four subunits are linked to each other by hydrogen bonds and hydrophobic interaction.
Because the four subunits are so closely linked, the hemoglobin tetramer is called a
molecule, even though no covalent bonds occur between the peptide chains of the four
subunits. In other proteins, the subunits are bound to each other by covalent bonds
(disulfide bridges.

15. PROTEIN DENATURATION
 When a solution of a protein is boiled, the protein frequently becomes insoluble—i.e., it is
denatured—and remains insoluble even when the solution is cooled.
 The denaturation of the proteins of egg white by heat—as when boiling an egg—is an
example of irreversible denaturation.
 The denatured protein has the same primary structure as the original, or native, protein.
 The weak forces between charged groups and the weaker forces of mutual attraction of
nonpolar groups are disrupted at elevated temperatures, however; as a result, the tertiary
structure of the protein is lost.
 In some instances the original structure of the protein can be regenerated; the process is
called renaturation.

16. REFERENCES
Stipanuk M.H.. “Biochemical and physiological aspects of human nutrition” W.B. Saunders
Company-An imprint of Elsevier Science, 2000
Mahan LK, Escott-Stump S.: “Krause’s foods, nutrition, and diet therapy” 10th ed. 2000
EmeritusProfessorof Biochemistry,Universityof California,Berkeley.Editor, Sciencemagazine,1985–
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